UV-stabilizers for siloxane systems

ABSTRACT

A UV-stabilizer comprising a mixture of hydroxybenzotriazole and a hydrolyzable silane containing at least one epoxy group is disclosed. The stabilizer is especially suitable as a component of a Siloxane system for coating substrates, especially thermoplastic substrates and most especially polycarbonate substrates.

The present invention relates to non-volatile UV-stabilising mixturesfor siloxane lacquer systems which mixtures have certainhydroxybenzotriazoles as the UV-stabilising active structure and whichare thus particularly suitable for the UV-stabilisation ofthermoplastics, in particular of aromatic polycarbonates.

Materials are frequently protected from the harmful influences of theenvironment by providing them with a protective surface. Siloxane-basedlacquers have proved particularly suitable for this purpose, inter aliaproviding the materials with a scratch-resistant surface.

These lacquers may contain so-called UV-stabilising substances in orderto protect the lacquer itself and the underlying material, thesubstrate, from harmful UV radiation. Apart from providing long-term UVprotection, one requirement placed upon these substances is, inter alia,that they are not volatile so that they remain homogeneously distributedwithin the lacquer layer and do not escape from the lacquer layer eitherduring curing or during subsequent use. The UV-stabilising substancesmust furthermore not decompose rapidly, must be durably homogeneouslymiscible with the lacquers and the lacquer containing the UV-stabilisingsubstances should be transparent.

U.S. Pat. Nos. 4,278,804 and 4,051,161 relate to UV-stabilising activesubstances and lacquers containing them. The substances disclosedtherein, however, exhibit the disadvantage that they provide inadequateUV protection, they decompose too rapidly and/or the siloxane systemcontaining the stabilisers has a yellow tinge.

U.S. Pat. No. 5,438,142 furthermore discloses the UV-stabilising activesubstance,1-(3′-(benzotriazol-2″-yl)-4′-hydroxyphenyl)-1,1-bis(4-hydroxyphenyl)ethane.This active substance, however, exhibits the disadvantage that it is notdurably miscible with siloxane-based lacquers.

The object thus arises of providing a UV-stabiliser system which doesnot exhibit the above-stated disadvantages.

This object is achieved according to the invention by the provision ofUV-stabilising mixtures containing hydroxybenzotriazole of the generalformula (1) below and hydrolysable silanes containing epoxy groups.

The present invention furthermore provides UV-stabilising mixtureshaving a molar ratio of epoxy groups of the silane to thehydroxybenzotriazole of the general formula (1) which is greater than2.5. preferably greater than 4, particularly preferably greater than 16.The molar ratio of epoxy units of the silane to the hydroxybenzotriazoleof the general formula (1) should not, however, exceed 1:200.

The mixtures according to the invention are suitable for theUV-stabilisation of siloxane systems, in particular of scratch- andabrasion-resistant siloxane coating materials. Such UV-stabilisedcoating materials, preferably lacquers, may be used for coatingmaterials of all kinds, such as for example wood, textiles, paper, stonearticles, but preferably for coating plastics, metals, glass andceramics, particularly preferably for coating thermoplastics and veryparticularly preferably for coating polycarbonates.

The hydroxybenzotriazoles used for the non-volatile, UV-stabilisingmixtures according to the invention are compounds of the general formula(1).

Preferably, however,1-(3′-(benzotriazol-2″-yl)-4′-hydroxyphenyl)-1,1-bis(4-hydroxyphenyl)ethane(hereinafter abbreviated to THPE-BZT) is used. The production ofTHPE-BZT is described in detail in U.S. Pat. No. 5,438,142, wherein thegeneral synthetic pathway is disclosed in scheme 1 of said patent. Theproduct is commercially obtainable from the company Hoechst/Celanese.

Silanes containing epoxy groups are generally taken to mean compoundswhich, on the one hand, possess at least one epoxy ring andsimultaneously have groups which form silanol structures underhydrolysing conditions.

Epoxysilanes as are preferably used according to the invention aredescribed, for example, in U.S. Pat. No. 9,946,701. They are compoundsof the general formulae (2) or (3):

R⁵ is a divalent hydrocarbon residue at most 9 carbon atoms or adivalent residue having at most 9 carbon atoms consisting of C, H and Oatoms, wherein the O atom is present as an ether bond residue.

m is 0 or 1.

Production of these epoxysilanes is also described in U.S. Pat. No.2,946,701. Reference is accordingly made to said patent. Particularlypreferred epoxysilanes are those compounds in which R₆ is methyl orethyl. They are commercially available, inter alia from the companiesUnion Carbide and Hüls AG as:

A-187 or 3-glycidyloxypropyltrimethoxysilane Dynasilan Glymo A-1862-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.

Production of the UV-stabilising Mixtures

The UV-stabilising mixtures are produced by homogeneously mixinghydroxy-benzotriazole of the general formula 1 with the hydrolysablesilanes containing epoxy groups and heating this mixture. Heating shouldbe performed for at least 30 minutes at at least 90° C. The temperatureshould preferably be above 120° C. during heating.

It has proved particularly favourable to use a mixing ratio at whichstoichiometrically more epoxy groups are present than the two freephenolic OH groups not required for UV-stabilisation of thehydroxybenzotriazoles of the general formula (1). The molar ratio ofepoxy units of the silane to hydroxybenzotriazole of the general formula1 should thus be greater than 2.5, preferably greater than 4,particularly preferably greater than 16.

The UV-stabilising components need not necessarily be producedseparately so that they may subsequently be added to the siloxane systemto be stabilised, but may also be synthesised in situ as a sub-stageduring synthesis of the siloxane systems/siloxane coating materials.

Siloxane Systems/Siloxane Coating Materials

The siloxane systems are substantially thermally curing systems whichpreferably crosslink by a condensation reaction to yield —Si—O—Si—linkages. Other crosslinking mechanisms may proceed in parallel. Suchsystems are described, for example, in U.S. Pat. Nos. 3,790,527,3,865,755, 3,887,514, 4,243,720, 4,278,804, 4,680,232, 4,006,271,4,476,281, in DE-A 4 011 045, 4 122 743, 4 020 316, 3 917 535, 3 706714, 3 407 087, 3 836 815, 2 914 427, 3 135 241, 3 134 777, 3 100 532, 3151 350, in DE-A 3 005 541, 3 014 411, 2 834 606, 2 947 879, 3 016 021,2 914 427 and 4 338 361 and should be considered part of the presentdisclosure.

The present invention also provides siloxane systems UV-stabilisedaccording to the invention.

Preferably used siloxane systems are those containing particulatematerial selected from among oxides, oxide hydrates, nitrides andcarbides of Si, Al, Sb and B and of transition metals, preferably Ti,Ce, Fe and Zr, and having a particle size in the range from 1 to 100 nm,preferably from 2 to 50 nm.

The UV-stabilising mixture according to the invention should be added tothe siloxane system in such a quantity, relative to the solids contentof the siloxane system, that the proportion of hydroxybenzotriazole,relative to the solids content of the siloxane system, is 0.3 to 20,preferably 3 to 15, particularly preferably 5 to 10 wt. %.

Reference is made to DE-A 2 914 427 and DE-A 4 338 361 with regard toproduction of siloxane-based scratch-resistant coating systems andcomponents thereof and these documents are thus part of the presentdescription.

Substrates, Materials

The siloxane systems provided with the UV-stabilising mixture accordingto the invention may be used as bulk materials and as coating materials.There are no restrictions as to the substrate materials which may beselected for coating. These UV-stabilised coating materials arepreferably suitable for coating wood, textiles, paper, stone articles,metals, glass, ceramics and plastics and in particular for coatingthermoplastics, as are described in Becker/Braun Kunststoffhandbuch,Carl Hanser Verlag, Munich, Vienna, 1992. They are very particularlysuitable for coating transparent thermoplastics, preferablypolycarbonates.

Conventional coating processes are used for coating purposes, forexample dipping, flooding, pouring, spinning, spraying or brushing.

The coating is applied to film thicknesses of, for example, 2 to 200 μm,preferably of 2 to 30 μm and particularly preferably of 5 to 15 μm. Thesubstrate may optionally be primed with a coupling agent or primer coatbefore application of the coating.

The lacquers are preferably cured at temperatures of >90° C.

For the purposes of the present invention, thermoplastic, aromaticpolycarbonates include both homopolycarbonates and copolycarbonates; thepolycarbonates may, in a known manner, be linear or branched.

A proportion, up to 80 mol %, preferably of 20 mol % to 50 mol %, of thecarbonate groups in the suitable polycarbonates may be replaced byaromatic dicarboxylic acid ester groups. Such polycarbonates, whichcontain both acid residues of carbonic acid and acid residues ofaromatic dicarboxylic acids incorporated in the molecular chain, aremore accurately termed polyester carbonates. They are to be subsumedwithin the superordinate term of thermoplastic, aromatic polycarbonates.

Details of the production of polycarbonates have been described inhundreds of patents over the past approx. 40 years. Reference is made,merely by way of example, to “Schnell, Chemistry & Physics ofPolycarbonates”, Polymer Reviews, volume 9, Interscience Publishers, NewYork, London, Sydney 1964, to D. C. Prevorsek, B. T. Debona & Y. Kesten,Corporate Research Center, Allied Chemical Corporation, Morristown, N.J.07960, “Synthesis of poly(ester carbonate) copolymers” in Journal ofPolymer Science, Polymer Chemistry edition, volume 19, 75-90 (1980), toD. Freitag, U. Grigo, P. R. Müller, N. Nouvertne', Bayer A G,“Polycarbonates” in Encyclopedia of Polymer Science & Engineering,volume 11, second edition, 1988, pages 648-718 and finally to Dr. U.Grigo, Dr. K. Kircher & Dr. P. R. Müller “Polycarbonate” inBecker/Braun, Kunststoff-Handbuch, volume 3/1, Polycarbonate,Polyacetale, Polyester, Celluloseester, Carl Hanser Verlag, Munich,Vienna, 1992, pages 117-299.

The thermoplastic polycarbonates have average molecular weights{overscore (M)}_(w) (determined by measuring relative viscosity at 25°C. in CH₂Cl₂ and a concentration of 0.5 g per 100 ml of CH₂Cl₂) of 12000to 400000, preferably of 18000 to 80000 and in particular of 22000 to60000.

The present invention accordingly also provides coated materials,preferably polycarbonate and particularly preferably polycarbonateprovided with a scratch-resistant coating.

EXAMPLES Example 1

a) UV-stabilising Mixture of THPE-BZT and3-glycidyloxypropyltrimethoxysilane (Glymo)

50 g of THPE-BZT and 450 g of 3-glycidyloxypropyltrimethoxysilane areintroduced into a vessel and heated to 140 to 150° C. under a nitrogenatmosphere while being stirred and are maintained at this temperaturefor one hour.

This mixture (mixture 1) was varied as follows:

Mixture: 2 100 g THPE-BZT 400 g Glymo 3 150 g THPE-BZT 350 g Glymo 4 200g THPE-BZT 300 g Glymo 5 100 g THPE-BZT 400 gα-(3,4-epoxycyclohexyl)ethyl- trimethoxysilane

b) Production of the Siloxane Coating Material According to DE-OS 2 914427 (Coating Sol I)

α) 19.8 g of glacial acetic acid, 210 g of distilled water and 227 g ofisopropanol are added to 300 g of colloidal silica having an SiO₂content of 30 wt. %. After thorough mixing, 900 g ofmethyltriethoxysilane are added and the mixture heated to 60° C. whilebeing stirred. The mixture is left at this temperature for 4 hours andthen a further 1200 g of isopropanol are added to the mixture. Once theproduct has cooled to room temperature, the slightly opaque solution isfiltered.

β) 340 g of isopropanol, 190 g of tetraethoxysilane and 360 g ofmethyltriethoxysilane are introduced into a vessel fitted with a stirrerand reflux condenser. This mixture is combined with 180 g of 0.05 nhydrochloric acid and co-hydrolysed by refluxing for five hours. Themixture is cooled to room temperature after the reaction. A solution isobtained which is a partial hydrolysate of tetraethoxysilane (5.1%,calculated as SiO₂) and a partial hydrolysate of methyltriethoxysilane(12.6%, calculated as CH₃SiO_(1.5)).

Before use as a coating material, the two components α) and β) are mixedtogether in a 1:1 ratio and dissolved in a mixture prepared from 60parts by weight of n-butanol, 40 parts by weight of acetic acid and 20parts by weight of toluene.

c) Production of a Siloxane Coating Material According to DE-OS 4 338361 (Coating Sol II)

A boehmite sol was produced by combining 12.82 g of aceticacid-stabilised (6.4 wt. % acetic acid) boehmite powder with 104.62 g of0.1 n HCl. Subsequent ultrasonication (20 minutes) produced atransparent, colourless solution, 24.3 g of which were combined with amixture prepared from 118.17 g of GPTS(3-glycidyloxypropyltrimethoxysilane) and 62.50 g of TEOS (tetraethylorthosilicate). The reaction mixture was stirred for 2 hours at roomtemperature and then, while being cooled with ice, combined with 18.93 gof aluminium tributoxyethanolate. The resultant clear sol was stirredfor 2 hours at room temperature and then, while being cooled with ice,combined with 93.14 g of the above boehmite sol and 79.30 g ofbutoxyethanol.

d) UV-stabilised Coating Sols I and II

A 75 g portion of the UV-stabilising mixture 2 according to theinvention was added to a 1000 g portion of each of coating sols I andII. Silica glass was coated with these compositions and UV lighttransmission measured with a Beckmann DU 70 photometer in the wavelengthrange from 250 to 600 nm. The coating film was 5 μm thick andabsorbed >98% of the radiation of a wavelength of <350 nm critical forpolycarbonate.

e) Coating of Substrates and Testing of Coating Properties

Bisphenol A polycarbonate sheets (T_(g)=147° C., {overscore (M)}_(w)27500) of dimensions 105×150×4 mm were cleaned with isopropanol andprimed by dipping in a mixture prepared from 3 wt. % ofaminopropyltrimethoxysilane and 97 wt. % of ethylene glycol monobutylether followed by 30 minutes' heat treatment at 130° C. The sheets werethen provided with a 20 μm film of one of coating sols I or II at adipping speed V=100 cm/min. After flashing off for 10 minutes at roomtemperature, the coated sheets were dried for 1 hour at 130° C. The filmthickness of the scratch-resistant lacquers was approx. 5 μm afterdrying. Once curing was complete, the coated sheets were stored for 2days at room temperature and then exposed to a defined quantity of UVradiation.

UV Exposure Testing

The polycarbonate sheets were exposed to filtered xenon arc radiationwith a water spray cycle to DIN 53387-1-A-X under the following testconditions:

Weathering apparatus: Xenon-WOM

Radiation intensity at 340 nm: 0.35 W/m² (preferably)

Filter combination: inner: Pyrex, outer: Pyrex

Blackboard temperature: 60° C.±5° C.

Black standard temperature: 65° C.±3° C.

Mode of operation: constant

Water spray cycle: 102:18

Relative atmospheric humidity: 60-80%

Yellowing as a function of exposure time was used as the evaluationcriterion for the weathering resistance of the lacquer-coated sheets.The corresponding yellowness of the sheets was determined as theYellowness Index (Y.I.) to ASTM D 1925-70.

Y.I. values after Xenon-WOM 102:18 weathering 0 h 1000 h 2000 h 3000 h5000 h Specimens Polycarbonate with 2.1 2.3 2.7 2.9 4.3 UV-stabilisedcoating sol I according to mixture 2 Polycarbonate with 2.4 2.6 3.1 3.24.9 UV-stabilised coating sol II according to mixture 2 ComparisonPolycarbonate according to 1.9 2.6 6.3^(a)) 7.9^(a)) —^(b)) Example 1ewith coating sol II without UV stabilisation ^(a))Cracks, delaminationof lacquer film. ^(b))No lacquer film remains.

What is claimed is:
 1. A process for producing a UV stabilizercomprising mixing and heating to at least 90° C. for at least 30 minutes(A) a hydroxybenzotriazole conforming to

 where R¹ denoted a member selected from the group consisting of H,C₁₈-alkyl, C₅₋₆-cycloalkyl and C₆₋₁₂-aryl, R² denotes a member selectedfrom the group consisting of H, halogen and C₁₋₁₂-alkyl, R³ denotes amember selected from the group consisting of H, C₁₋₁₂-alkyl, halogen andC₆₋₁₂-aryl and R⁴ is a member selected from the group consisting of H,C₁₋₁₈-alkyl and C₆₋₁₈-aryl, and (B) at least one hydrolyzable silanecontaining at least one epoxy group.
 2. The process of claim 1 whereinthe molar ratio of said at least one epoxy group to saidhydroxybenzotriazole is greater than 2.5.
 3. The process of claim 1wherein said (A) is1-(3′-(benzotriazol-2″-yl)-4′-hydroxyphenyl)-1,1-bis(4-hydroxyphenyl)ethane.4. A coated substrate prepared by the method of claim
 1. 5. TheUV-stabilizer produced in accordance with the process of claim
 1. 6. Acoating composition comprising siloxane and the UV-stabilizer of claim5.
 7. The coating composition of claim 6 wherein siloxane containssolids in the form of particulate material having a particle size in therange of 1 to 100 nm selected from a first group consisting of oxides,oxide hydrates, nitrides and carbides of at least one member selectedfrom a second group consisting of Si, Al, Sb, B and a transition metal.8. The coating composition of claim 7 wherein weight ratio ofhydroxybenzotriazole to said solids is 0.3 to
 20. 9. The coatingcomposition of claim 8 wherein second group consists of Ti, Ce, Fe andZr.
 10. A method of using the UV-stabilizer of claim 5 comprisingcoating at least a part of the surface of a substrate therewith.
 11. Themethod of claim 10 wherein substrate is thermoplastic.
 12. The method ofclaim 11 wherein thermoplastic is polycarbonate.